Tire comprising a tread formed by multiple elastomer blends

ABSTRACT

A tire with a radial carcass reinforcement, having a crown reinforcement, itself capped radially by a tread connected to two beads by two sidewalls, the tread having at least two layers of blended elastomeric compounds that are radially superposed and have a voids ratio that is lower in the central part than at the axially outer parts. A first layer of blended elastomeric compounds of the tread is made up of a first blended elastomeric compound forming a part extending at least into the region of the equatorial plane and of at least two axially outer parts formed of a second blended elastomeric compound, the first blended elastomeric compound having a macro dispersion Z-value higher than 65 and a maximum tan(δ) value, denoted tan(δ)max, lower than 0.150, and the second blended elastomeric compound having a maximum tan(δ) value, denoted tan(δ)max, lower than 0.130.

This application is a 371 national phase entry of PCT/EP2013/063540,filed 27 Jun. 2013, which claims benefit of French Patent ApplicationNo. 1256489, filed 5 Jul. 2012, the entire contents of which areincorporated herein by reference for all purposes.

BACKGROUND

1. Field

The disclosure relates to a tire with a radial carcass reinforcementintended to be fitted to a heavy vehicle such as a transport vehicle oran “industrial” vehicle. It is notably a tire that has an axial widthgreater than 37 inches.

Although not limited to this type of application, the disclosure will bemore particularly described with reference to a vehicle of the “dumper”type weighing in excess of 300 tonnes, fitted with tires of a diametergreater than three meters fifty with an axial width greater than 37inches.

2. Description of Related Art

Such a tire, intended in general to carry heavy loads, comprises aradial carcass reinforcement and a crown reinforcement made up of atleast two working crown plies formed of inextensible reinforcingelements which are crossed from one ply to the next and make equal orunequal angles of between 10 and 45° with the circumferential direction.

As far as the usual design of tires for industrial vehicles isconcerned, the radial carcass reinforcement, which is anchored in eachbead, is made up of at least one layer of metal reinforcing elements,the said elements being substantially parallel to one another within thelayer. The carcass reinforcement is usually surmounted by a crownreinforcement made up of at least two working crown layers of metalreinforcing elements, these however being crossed from one layer to thenext and making angles of between 10 and 65° with the circumferentialdirection. Between the carcass reinforcement and the working crownlayers there are usually two layers of reinforcing elements, crossed oneply to the next and at angles smaller than 12°; the width of theselayers of reinforcing elements is usually less than those of the workinglayers. Radially on the outside of the working layers there are alsoprotective layers the reinforcing elements of which are at angles ofbetween 10 and 65°.

Radially on the outside of the crown reinforcement is the tread usuallymade up of polymeric materials intended to come into contact with theground in the contact patch in which the tire is in contact with theground.

Cords are said to be inextensible when the said cords exhibit, under atensile force equal to 10% of the breaking force, a relative elongationat most equal to 0.2%.

Cords are said to be elastic when the said cords exhibit, under atensile force equal to the breaking load, a relative elongation at leastequal to 3% with a maximum tangent modulus of less than 150 GPa.

The circumferential direction of the tire, or longitudinal direction, isthe direction corresponding to the periphery of the tire and defined bythe direction of running of the tire.

The axis of rotation of the tire is the axis about which it turns innormal use.

A radial or meridian plane is a plane containing the axis of rotation ofthe tire.

The circumferential median plane, or equatorial plane, is a planeperpendicular to the axis of rotation of the tire and which divides thetire into two halves.

The transverse or axial direction of the tire is parallel to the axis ofrotation of the tire. An axial distance is measured in the axialdirection. The expression “axially on the inside of or, respectively,axially on the outside of” means “of which the axial distance, measuredfrom the equatorial plane, is less than or, respectively, greater than”.

The radial direction is a direction that intersects the axis of rotationof the tire and is perpendicular thereto. A radial distance is measuredin the radial direction. The expression “radially on the inside of or,respectively, radially on the outside of” means “of which the radialdistance, measured from the axis of rotation of the tire, is less thanor, respectively, greater than”.

In the case of vehicles, notably those intended for use in mines orquarries for transporting loads, the difficulties regarding accessroutes and profitability requirements lead the manufacturers of thesevehicles to increase their load-carrying capability. It then followsthat the vehicles become increasingly large and, therefore, increasinglyheavy themselves so that they can transport an increasingly heavy load.At the present time, these vehicles may reach weights of severalhundreds of tonnes and so may the weight of the load to be transported;the overall weight may therefore be as much as 600 tonnes.

Increasing demands are therefore placed on the tires. The tires havesimultaneously to exhibit good performance in terms of wear, be capableof transporting the necessary torque and of withstanding the attacknotably from rocks encountered on the tracks.

Document FR 1445678 thus proposes choosing different materials in theaxial direction for forming the tread. The central part may be made upof a more wear-resistant material than the lateral parts.

Wear-resistant materials usually carry a penalty in terms of hysteresisproperties. Thus it is also known practice for the tread of a tire to bemade of a radial superposition of two different materials in order toreach a compromise between wear properties and hysteresis propertiesthat is satisfactory for the envisaged applications.

Such tires are, for example, described in document U.S. Pat. No.6,247,512. That document describes the superposition of two layers ofmaterials in order to form the tread, the outer material coming intocontact with the ground notably exhibiting better performance in termsof wear whereas the inner material has hysteresis properties that makeit possible to limit increases in tire temperature in the crown region.

With this type of tire, the patterns present on the tread may also varyin the axial direction; thus it is known practice to have a lower voidsratio in the central part in order to transmit torque, and also avoidattack in the central part where rocks are more difficult to remove.

The voids ratio of the tread is defined according to the invention bythe ratio of the surface area of sipes or grooves to the total area ofthe tread. The voids ratio of a part of the tread is thus definedaccording to the invention by the ratio of the surface area of sipes orgrooves present in the said part of the tread, to the total surface areaof the said part of the tread.

SUMMARY

The inventors set themselves the task of providing tires with an evenbetter compromise between the performance aspects of wear and endurance.

According to an embodiment of the invention, this object has beenachieved using a tire with a radial carcass reinforcement, comprising acrown reinforcement, itself capped radially by a tread connected to twobeads by two sidewalls, the tread comprising at least two layers ofblended elastomeric compounds that are radially superposed and have avoids ratio that is lower in the central part than at the axially outerparts, a first layer of blended elastomeric compounds of the tread beingmade up of a first filled blended elastomeric compound forming a partextending at least into the region of the equatorial plane and of atleast two axially outer parts formed of a second blended elastomericcompound, the first filled blended elastomeric compound having a macrodispersion Z-value higher than 65 and a maximum tan(δ) value, denotedtan(δ)max, lower than 0.150, and the second blended elastomeric compoundhaving a maximum tan (δ) value, denoted tan (δ) max, lower than 0.130.

A macro dispersion Z-value higher than 65 for a filled blendedelastomeric compound means that the filler is dispersed in theelastomeric matrix of the composition with a dispersion Z-value of 65 orhigher.

In the present description, the dispersion of filler in an elastomericmatrix is characterized by the Z-value which is measured, aftercrosslinking, using the method described by S. Otto et al in KautschukGummi Kunststoffe, 58 Jahrgang, N R 7-8/2005, which is consistent withISO standard 11345.

The Z-value calculation is based on the percentage of surface area inwhich the filler is not dispersed (“non-dispersed % area”) as measuredby the “disperGRADER+” device supplied with its operating procedure and“disperDATA” exploitation software by the company Dynisco, using theequation:

Z=100−(non-dispersed % area)/0.35

The non-dispersed percentage area is itself measured using a camera thatlooks at the surface of the specimen under light incident at an angle of30°. Light-colored points are associated with filler and agglomerations,while dark points are associated with the rubber matrix; a digitalprocessing operation converts the image into a black-and-white image andallows the percentage of non-dispersed area to be determined, in the waydescribed by S. Otto in the abovementioned document.

The higher the Z-value, the better the dispersion of the filler in therubber matrix (a Z-value of 100 corresponding to a perfect dispersionand a Z-value of 0 to a mediocre dispersion). A Z-value of 65 or higheris considered to correspond to a good dispersion of the filler in theelastomeric matrix.

The blended elastomeric compounds of which the tread is made up areprepared using known methods.

In order to achieve a macro dispersion Z-value higher than 65, theblended elastomeric compound that makes up the radially outer part mayadvantageously be prepared by forming a master batch of diene elastomerand reinforcing filler.

Within the meaning of the disclosure a “master batch” means anelastomer-based composite into which a filler has been introduced.

There are various ways of obtaining a master batch of diene elastomerand reinforcing filler. In particular, one type of solution is, in orderto improve the dispersion of the filler in the elastomer matrix, to mixthe elastomer and the filler in the “liquid” phase. To do that, use ismade of an elastomer in the form of a latex which takes the form ofparticles of elastomer dispersed in water, and of an aqueous dispersionof the filler, namely a filler dispersed in water and commonly referredto as a “slurry”.

Thus, according to one of the alternative forms of the invention, themaster batch is obtained by liquid-phase mixing from a diene elastomerlatex containing natural rubber and an aqueous dispersion of a fillercontaining carbon black.

More preferably still, the master batch according to an embodiment ofthe invention is obtained using the following process steps, that makeit possible to obtain very good dispersion of the filler in theelastomer matrix:

-   -   supplying a continuous first stream of a diene elastomer latex        to a mixing zone of a coagulation reactor defining an elongate        coagulation zone extending between the mixing zone and an        outlet,    -   supplying the said mixing zone of the coagulation reactor with a        second continuous stream of a fluid containing a filler under        pressure in order to form a mixture with the elastomer latex by        mixing the first fluid and the second fluid in the mixing zone        vigorously enough to cause the elastomer latex to coagulate with        the filler before the outlet, the said mixture flowing as a        continuous stream towards the outlet zone and the said filler        being capable of coagulating the elastomer latex,    -   collecting at the outlet to the reactor the coagulum obtained        previously in the form of a continuous stream and drying it in        order to recover the master batch.

A method of preparing a master batch in the liquid phase is describedfor example in document WO 97/36724.

The loss factor tan(δ) is a dynamic property of the layer of blendedrubber compound. It is measured using a viscoanalyser (Metravib VA4000)in accordance with standard ASTM D 5992-96. The response of a sample ofvulcanized composition (cylindrical test specimen 4 mm thick and 400 mm²in cross section), subjected to simple alternating sinusoidal shearstressing at a frequency of 10 Hz, and a temperature of 100° C. isrecorded. The amplitude of deformation sweeps from 0.1 to 50% (in theoutbound cycle) and then from 50% to 1% (in the return cycle). Theresults exploited are the complex dynamic shear modulus (G*) and theloss factor tan(δ). In the return cycle, the maximum observed value oftan(δ), denoted tan(δ)_(max) is indicated.

The rolling resistance is the resistance that arises when the tire isrolling and indicates the rising temperature of the tire. It is thusrepresented by hysteresis losses associated with the deformation of thetire over one revolution. The tan(δ) values for the materials used aremeasured at 10 Hz between 30 and 100° C. in order to incorporate theeffect of the various frequencies of deformation caused by the turningof the tire. The value of tan(δ) at 100° C. thus corresponds to anindication of the rolling resistance of the tire during running.

The tearability indexes are measured at 100° C. In particular, the forcethat has to be exerted in order to obtain breakage (FRD) is determinedin N/mm of thickness and the elongation at rupture (ARD) is measured asa percentage on a test specimen measuring 10×105×2.5 mm notched at itscentre along its length over a depth of 5 mm.

The various measurements are taken on new tires that have not yet beenrun at all.

According to a preferred embodiment of the invention, the voids ratio ofthe central part of the tread is between 2 and 15% and, for preference,less than 10%.

For preference also, the voids ratio of the axially outer parts of thetread is between 20 and 40% and preferably less than 30%.

The inventors have been able to demonstrate that the combination of afirst filled blended elastomeric compound having a macro dispersionZ-value higher than 65 and a maximum tan (δ) value, denoted tan (δ) max,lower than 0.150, by way of material for the central part of the firstlayer of the tread coinciding at least in part with the region with thelowest voids ratio and of a second blended elastomeric compound having amaximum tan (δ) value, denoted tan (δ) max, lower than 0.130, by way ofmaterial for the axially outer parts of the tread coinciding at least inpart with the region of highest voids ratio leads to a compromisebetween wear resistance and endurance.

Specifically, as mentioned previously, the voids ratio in the middle ofthe tread is lower by comparison with the axially outer parts. Thisreduction in the voids ratio in the central part of the tread makes itpossible to safeguard the crown reinforcement from attack, it notablybeing a tricky business to remove stones from this central part of thetread.

Next, choosing the first filled blended elastomeric compound with amacro dispersion Z-value higher than 65 as the material for the centralpart of the tread leads to wearing properties that are particularlyadvantageous in terms of the use of such tires, the inventors havingbeen able to demonstrate that the most pronounced wear occurs in thecentral part of the tread, which is the part most heavily loaded whentransmitting torque.

Next, choosing the first filled blended elastomeric compound as thematerial for the central part of the tread, having a tan (δ) max valuelower than 0.150, and choosing the second filled blended elastomericcompound as the material for the axially outer parts of the tread,having a tan (δ) max value lower than 0.130, lead to hysteresisproperties that favor lower heating of the tire and thus better tireendurance.

Choosing these blended elastomeric compounds may also make it possibleto reduce the operating temperature of the tire in the crown region inorder to make it possible to lower the rolling resistance of the tire.

According to a preferred embodiment of the invention, the first blendedelastomeric compound forming a part that extends at least into theregion of the equatorial plane of the first layer of blended elastomericcompounds contains, by way of reinforcing filler, at least a carbonblack with a BET specific surface area higher than 120 m²/g, used in aproportion of between 10 and 70 phr.

For preference also, the reinforcing filler of the first blendedelastomeric compound forming a part that extends at least into theregion of the equatorial plane of the first layer of blended elastomericcompounds contains by way of reinforcing filler a cut of carbon black asdescribed hereinabove and of a white filler, the overall level of fillerbeing between 10 and 90 phr and the ratio of carbon black to whitefiller being higher than 2.7.

The selection of the fillers as described hereinabove makes it possiblefurther to improve the wear resistance properties of the first blendedelastomeric compound of the first layer of blended elastomericcompounds.

Advantageously also according to an embodiment of the invention, thecomplex dynamic shear modulus G* 1% at 100° C. of the first blendedelastomeric compound forming a part that extends at least into theregion of the equatorial plane of the first layer of blended elastomericcompounds is higher than 2.30.

According to a first embodiment of the invention, the tire according tothe invention comprising at least two radially superposed layers ofblended elastomeric compounds, the first layer as previously describedand comprising a first blended elastomeric compound forming a part thatextends at least into the region of the equatorial plane and at leasttwo axially outer parts formed of a second blended elastomeric compound,forms the radially outer layer of the tread, which layer is intended tocome into contact with the ground.

According to this first embodiment, the second layer of blendedelastomeric compound radially on the inside of and in contact with thesaid first layer is advantageously made up of a blended elastomericcompound having a tan (δ) max value lower than 0.130.

Advantageously also, the second layer of blended elastomeric compoundradially on the inside of and in contact with the said first layer ismade up of a blended elastomeric compound identical to the secondblended elastomeric compound forming the axially outer parts of thefirst layer of blended elastomeric compounds.

The presence of this second blended elastomeric compound radially on theinside of the first layer and notably of the central part thereof maymake it possible to further limit the heading of the tire and thus leadto even better tire endurance.

According to a second embodiment of the invention, the tire according tothe invention comprising at least two radially superposed layers ofblended elastomeric compounds, the first layer as previously described,and comprising a first blended elastomeric compound forming a partextending at least into the region of the equatorial plane and at leasttwo axially outer parts formed of a second blended elastomeric compound,forms a radially inner layer of the tread. Such a layer of the tread isintended to come into contact with the ground only after the said treadhas been worn away by a given amount that has caused the radiallyoutermost layer or layers to disappear.

According to this second embodiment, the radially outer second layer ofblended elastomeric compound preferably in contact with the said firstlayer is advantageously made up of a blended elastomeric compound havinga macro dispersion Z-value higher than 65 and a maximum tan(δ) value,denoted tan(δ)max; lower than 0.150.

Advantageously also, the radially outer second layer of blendedelastomeric compound in contact with the said first layer is made up ofa blended elastomeric compound identical to the first blendedelastomeric compound forming the central part of the said first layer ofblended elastomeric compounds.

The presence of this first blended elastomeric compound radially on theoutside of the first layer and notably of the axially outer partsthereof may make it possible to improve the wear resistance propertiesof the tread. The inventors have been able to demonstrate that undercornering, the axially outer parts of the tires of industrial vehiclesof the dumper type with an axial width greater than 37 inches aresubjected to particularly severe stress loadings. It also proves to bethe case that the severity of these stress loadings varies with tiresize, stress loadings notably increasing in line with the ratio of sidewall height to tire width.

The thickness of this second layer of blended elastomeric compoundaccording to this second embodiment of the invention may thusadvantageously vary according to the size of the tire. According to anembodiment of the invention, the thickness of this second layer formingthe radially outer part of the tread advantageously decreases as tirewidth increases. It is furthermore advantageously defined so that itdisappears following wear that has led to an appreciable reduction inthe stress loadings leading to reduced mobility of the tread blocks inthe pattern and an increase in the transverse rigidity of the tire whichare favorable under cornering.

For tires of 57 inches or less (diameter measured between the base ofthe beads of the tire), the thickness, measured in the radial directionat the end of what in a meridian section of the tire is the radiallyoutermost working layer, of the second layer of blended elastomericcompound according to this second embodiment of the invention ispreferably greater than 50% of the thickness, measured in the radialdirection at the end of what in a meridian section of the tire is theradially outermost working layer, of the first layer of blendedelastomeric compound.

For tires larger than 57 inches (diameter measured between the base ofthe beads of the tire), the thickness, measured in the radial directionat the end of what in a meridian section of the tire is the radiallyoutermost working layer, of the second layer of blended elastomericcompound according to this second embodiment of the invention ispreferably greater than 10% of the thickness, measured in the radialdirection at the end of what in a meridian section of the tire is theradially outermost working layer, of the first layer of blendedelastomeric compound.

For preference also according to this second embodiment of theinvention, the tread comprises a third layer of blended elastomericcompound radially on the inside of and in contact with the said firstlayer, advantageously made up of a blended elastomeric compound having atan (δ) max value lower than 0.130.

Advantageously also, the third layer of blended elastomeric compoundradially on the inside of and in contact with the said first layer ismade up of a blended elastomeric compound identical to the secondblended elastomeric compound forming the axially outer parts of the saidfirst layer of blended elastomeric compounds.

As in the first embodiment, such a radially inner layer makes itpossible to have a blended elastomeric compound that makes it possibleto limit the heading of the tire and thus improve the endurance thereof.

Because the overall thickness of the tread is defined by parametersconnected with the desire and use of the tires, the thickness of thevarious layers that make up the tread has to vary with the size of thetire as mentioned previously.

For tires of 57 inches or less (diameter measured between the base ofthe beads of the tire), the thickness, measured in the radial directionat the end of what in a meridian section of the tire is the radiallyoutermost working layer, of the third layer of blended elastomericcompound according to this second embodiment of the invention ispreferably greater than 25% of the thickness, measured in the radialdirection at the end of what in a meridian section of the tire is theradially outermost working layer, of the first layer of blendedelastomeric compound.

For tires larger than 57 inches (diameter measured between the base ofthe beads of the tire), the thickness, measured in the radial directionat the end of what in a meridian section of the tire is the radiallyoutermost working layer, of the third layer of blended elastomericcompound according to this second embodiment of the invention ispreferably greater than 35% of the thickness, measured in the radialdirection at the end of what in a meridian section of the tire is theradially outermost working layer, of the first layer of blendedelastomeric compound.

One advantageous alternative form of the invention according to eitherone of the embodiments mentioned hereinabove, also provides for thepresence of an additional layer of a blended elastomeric compound in aradially innermost position in the tread and therefore in contact withthe crown reinforcement of the tire, having a maximum tan(δ) value,denoted tan(δ)max, lower than 0.100.

The presence of this additional layer radially furthest towards theinside of the tread may make it possible to reduce still further theincreases in tread temperature and therefore favor the endurance of thetire.

The presence of this additional layer radially furthest towards theinside of the tread may also make it possible to reduce the operatingtemperature of the tire in the crown region in order to make it possibleto lower the rolling resistance of the tire.

The thickness of this additional layer of blended elastomeric compound,measured in the radial direction at the end of the working layer that isradially outermost in a meridian section of the tire, is advantageouslybetween 15% and 25% of the thickness of the complete tread measured, inthe radial direction at the end of the working layer that is radiallyoutermost in a meridian section of the tire.

According to an embodiment of the invention, the aforementionedthickness measurements are taken from tires in the new state, which havenot been run and therefore exhibit absolutely no tread wear.

BRIEF DESCRIPTION OF DRAWINGS

Further details and advantageous features of the invention will becomeapparent hereinafter from the description of some embodiments of theinvention given with reference to FIGS. 1 to 3 which depict:

FIG. 1, a diagrammatic meridian section of a tire according toembodiments of the invention,

FIG. 2, a diagrammatic meridian view of the tread of the tire of FIG. 1,according to a first embodiment of the invention,

FIG. 3, a diagrammatic meridian view of the tread of the tire of FIG. 1,according to a second embodiment of the invention.

For ease of understanding, the figures have not been drawn to scale.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 schematically depicts a tire 1 intended to be used on vehicles ofthe dumper type. It comprises a radial carcass reinforcement 2 anchoredin two beads 3 around bead wires 4. The carcass reinforcement 2 isformed of a layer of metal cords. The carcass reinforcement 2 is hoopedby a crown reinforcement 5, itself capped by a tread 6. The tread 6 is,according to the invention, made up of a central part 7 extending atleast into the region of the equatorial plane XX′, and of two axiallyouter parts 8, 9.

According to an embodiment of the invention, the central part 7 of thetread 6 has a voids ratio (the tread patterns are not depicted in thefigures) which is lower than that of the axially outer parts 8 and 9.

FIGS. 2 and 3 very schematically illustrate the makeup of the tread 6for two tires of different sizes, the tread 6 being, according to anembodiment of the invention, made up of at least two radially superposedlayers of blended elastomeric compound, a first layer of blendedelastomeric compound being made up of a first blended elastomericcompound forming a part extending at least in the region of theequatorial plane and of at least two axially outer parts formed of asecond blended elastomeric compound.

FIG. 2 schematically depicts the tread 6 of a tire of size 53/80R63.

The voids ratio in the central part of the tread 6 is 3%. The voidsratio in the axially outer parts of the tread 6 is 29%.

According to an embodiment of the invention, the tread 6 is made up of afirst layer 61 formed of a first blended elastomeric compound M1 forminga part 61 a extending at least into the region of the equatorial planeXX′ and of at least two axially outer parts 61 b formed of a secondblended elastomeric compound M2.

The filled blended elastomeric compound M1 has a macro dispersionZ-value of 80 and a tan (δ) max value of 0.133.

The blended elastomeric compound M2 has a tan (δ) max value of 0.122.

The tread 6 comprises a radially outer second layer 62 that comes intocontact with the ground and is made up of the compound M1.

The tread 6 further comprises a radially inner third layer 63 in contactwith the first layer 61 made up of the blended compound M2.

The tread 6 also comprises a radially innermost additional layer 64 madeup of a blended elastomeric compound M3 with a tan(δ)max value of 0.060.

The compounds M1, M2 and M3 are described in the table below, togetherwith a number of their properties.

Compound Compound Compound M1 M2 M3 NR 100 100 100 Black with BETspecific area 46 150 m²/g and structure OAN 135 ml/100 g Black N115 40Black N347 34 Silica 170G 10 15 10 Anti-ozone wax C32 ST 1 1 Antioxidant(6PPD) 1.5 1.5 1 Silane on black 2 PEG (6000-20000) 1.67 2.5 Stearicacid 1 2 2 Accelerant CBS 1.7 1.7 1.35 Sulphur sol 2H 1.2 1.2 1.45 Zincoxide 2.7 3 4.5 Z value 80 54 40 tan(δ)_(max) 0.133 0.122 0.060 G*1%return 2.57 2.13 2.00

The thickness d₆₁ of the first layer 61 is 64 mm.

The thickness d₆₂ of the second layer 62 is 14 mm.

The thickness d₆₃ of the third layer 63 is 28 mm.

The thickness d₆₄ of the additional layer 64 is 22 mm.

The ratio of the thickness d₆₂ of the second layer 62 to the thicknessd₆₁ of the first layer 61 is 22% and therefore higher than 10%.

The ratio of the thickness d₆₃ of the third layer 63 to the thicknessd₆₁ of the first layer 61 is 44% and therefore higher than 35%.

The ratio of the thickness d₆₄ of the additional layer 64 to the totalthickness of the tread, namely the sum of the thicknesses(d₆₁+d₆₂+d₆₃+d₆₄) is 17% and therefore between 15 and 25%.

The thicknesses are measured on a meridian section of a tire in the newstate, in the radial direction at the end of the radially outermostworking layer.

FIG. 3 schematically depicts the tread 6 of a tire of size 40.00R57.

The voids ratio in the central part of the tread 6 is 3%. The voidsratio in the axially outer parts of the tread 6 is 33%.

According to an embodiment of the invention, the tread 26 is made up ofa first layer 261 formed of a first blended elastomeric compound M21forming a part 261 a extending at least into the region of theequatorial plane XX′ and of at least two axially outer parts 261 bformed of a second blended elastomeric compound M22.

The tread 26 comprises a radially outer second layer 262 coming intocontact with the ground and made up of the compound M21.

The tread 26 also comprises a radially inner third layer 263 in contactwith the first layer 261 made up of the compound M22.

The tread 26 also comprises a radially innermost additional layer 264made up of a blended elastomeric compound M23 a tan(δ)max value of0.050.

The compounds M21, M22 and M23 are identical to the compounds M1, M2 andM3 corresponding to FIG. 2.

The thickness d₂₆₁ of the first layer 261 is 32 mm.

The thickness d₂₆₂ of the second layer 262 is 46 mm.

The thickness d₂₆₃ of the third layer 263 is 14 mm.

The thickness d₂₆₄ of the additional layer 264 is 22 mm.

The ratio of the thickness d₂₆₂ of the second layer 262 to the thicknessd₂₆₁ of the first layer 261 is 140% and therefore higher than 50%.

The ratio of the thickness of d₂₆₃ of the third layer 263 to thethickness d₂₆₁ of the first layer 261 is 44% and therefore higher than25%.

The ratio of the thickness d₆₄ of the additional layer 264 to the totalthickness of the tread, namely the sum of the thicknesses(d₂₆₁+d₂₆₂+d₂₆₃+d₂₆₄) is 19% and therefore between 15 and 25%.

As with FIG. 2, the thickness measurements are taken in a meridiansection of a tire in the new state, in the radial direction at the endof the radially outermost working layer.

Tests were carried out using vehicles fitted with the tires according tothe invention in order to evaluate the wearing properties thereof.

These tests involve running tires fitted to the driven rear axle of avehicle. The vehicles are driven along a track inclined at 14%successively uphill and downhill for a total duration of 1500 hours. Thetrack is made up of stones ranging in size between 15 and 30 mm.

These tests are carried out on the one hand with tires according to thedepiction of FIG. 2, denoted P1 (size 53/80R63) and tires according tothe depiction of FIG. 3, denoted P2 (size 40.00R57).

The tires P1 of size 53/80R63 are inflated to a pressure of 7 bar andsubjected to a load of 87.5 tonnes.

The tires P2 of size 40.00R57 are inflated to a pressure of 7 bar andsubjected to a load of 64.5 tonnes.

These tires are respectively compared against reference tires R1 and R2fitted respectively to the same vehicles. The tires R1 and R2 are tireswhich are known for this type of application.

The tires R1 are the same size as the tires P1 and the tires R2 are thesame size as the tires P2.

The treads of the reference tires R1 and R2 are produced in identicalways and comprise two radially superposed layers, the radially outerlayer being made up of a first material A1 and the radially inner layerbeing made up of a material A2.

In tire R1, the thickness of the radially outer layer made up of thematerial A1 is 106 mm and the thickness of the radially inner layer madeup of the material A2 is 22 mm.

In tire R2, the thickness of the radially outer layer made up of thematerial A1 is 92 mm and the thickness of the radially inner layer madeup of the material A2 is 22 mm.

The makeup and properties of these materials are described in the tablebelow.

Compound A1 Compound A2 NR 100 100 Black N115 40 Black N347 34 Silica170G 15 10 Anti-ozone wax C32 ST 1 Antioxidant (6PPD) 1.5 1 Silane onblack 2 PEG (6000-20000) 2.5 Stearic acid 2 2 Accelerant CBS 1.4 1.35Sulphur sol 2H 1.4 1.45 Zinc oxide 3 4.5 Z value 57 40 tan(δ)_(max)0.142 0.060 G*1% return 2.72 2.00

The results obtained when running under the conditions describedhereinabove demonstrated improvements in wear of between 15 and 20% withtires according to an embodiment of the invention as compared againstthe reference tires.

The results show that the tires according to an embodiment of theinvention lead to a better compromise between the various wear andendurance performance aspects, notably as a result of heat dissipationbeneficial to the shoulders.

1. A tire with a radial carcass reinforcement, comprising: a crownreinforcement, itself capped radially by a tread radially capping thecrown reinforcement and connected to two beads by two sidewalls, thetread comprising at least two layers of blended elastomeric compoundsthat are radially superposed and have a voids ratio that is lower in acentral part than at axially outer parts, wherein a first layer ofblended elastomeric compounds of the tread is made up of a first blendedelastomeric compound forming a part extending at least into the regionof the equatorial plane and of at least two axially outer parts formedof a second blended elastomeric compound, wherein the first blendedelastomeric compound has a macro dispersion Z-value higher than 65 and amaximum tan(δ) value, denoted tan(δ)max, lower than 0.150, and whereinthe second blended elastomeric compound has a maximum tan(δ) value,denoted tan(δ)max, lower than 0.130.
 2. The tire according to claim 1,wherein the first blended elastomeric compound contains, by way ofreinforcing filler, at least carbon black used at a content of between10 and 70 phr, and wherein the carbon black has a BET specific surfacearea higher than 140 m²/g.
 3. The tire according to claim 1, wherein thefirst blended elastomeric compound contains, by way of a cut of carbonblack, with a BET specific surface area higher than 140 m²/g, and of awhite filler, wherein the reinforcing filler is used at a content ofbetween 10 and 90 phr, and wherein the ratio of carbon black to whitefiller is higher than 2.7.
 4. The tire according to claim 1, wherein thecomplex dynamic shear modulus G* 1% at 100° C. of the first blendedelastomeric compound is higher than 2.30.
 5. The tire according to claim1, wherein the first layer of blended elastomeric compound, made up of afirst blended elastomeric compound forming a part that extends at leastinto the region of the equatorial plane and of at least two axiallyouter parts formed of a second blended elastomeric compound, forms theradially outer layer of the tread.
 6. The tire according to claim 5,wherein the blended elastomeric compound that makes up the radiallyinner second layer in contact with the first layer has a maximum tan(δ)value, denoted tan(δ)max, lower than 0.130.
 7. The tire according toclaim 1, wherein the blended elastomeric compound that makes up aradially outer second layer in contact with the first layer, forming theradially outer layer of the tread, has a macro dispersion Z-value higherthan 65 and a maximum tan(δ) value, denoted tan(δ)max, lower than 0.150.8. The tire according to claim 7, wherein the tread comprises a radiallyinner third layer in contact with the said first layer, and wherein theblended elastomeric compound that makes up the said third layer has amaximum tan(δ) value, denoted tan(δ)max, lower than 0.130.
 9. The tireaccording to claim 7, wherein the tire is of a dimension less than orequal to 57 inches, wherein the thickness, measured in the radialdirection at the end of what in a meridian section of the tire is theradially outermost working layer, of the second layer of blendedelastomeric compound is greater than 50% of the thickness, measured inthe radial direction at the end of what in a meridian section of thetire is the radially outermost working layer, of the first layer ofblended elastomeric compound.
 10. The tire according to claim 8, whereinthe tire is of a dimension less than or equal to 57 inches, wherein thethickness, measured in the radial direction at the end of what in ameridian section of the tire is the radially outermost working layer, ofthe third layer of blended elastomeric compound is greater than 25% ofthe thickness, measured in the radial direction at the end of what in ameridian section of the tire is the radially outermost working layer, ofthe first layer of blended elastomeric compound.
 11. The tire accordingto claim 7, wherein the tire is of a dimension strictly greater than 57inches, wherein the thickness, measured in the radial direction at theend of what in a meridian section of the tire is the radially outermostworking layer, of the second layer of blended elastomeric compound isgreater than 10% of the thickness, measured in the radial direction atthe end of what in a meridian section of the tire is the radiallyoutermost working layer, of the first layer of blended elastomericcompound.
 12. The tire according to claim 8, wherein the fire is of adimension strictly greater than 57 inches, wherein the thickness,measured in the radial direction at the end of what in a meridiansection of the tire is the radially outermost working layer, of thethird layer of blended elastomeric compound is greater than 35% of thethickness, measured in the radial direction at the end of what in ameridian section of the tire is the radially outermost working layer, ofthe first layer of blended elastomeric compound.
 13. The tire accordingto claim 1, wherein the tire further comprises an additional layer of ablended elastomeric compound in a radially innermost position of thetread, and wherein the blended elastomeric compound of the saidadditional layer has a maximum tan(δ) value, denoted tan(δ)max, lowerthan 0.100.
 14. The tire according to claim 8, wherein the tire is of adimension less than or equal to 57 inches, wherein the thickness,measured in the radial direction at the end of what in a meridiansection of the tire is the radially outermost working layer, of thesecond layer of blended elastomeric compound is greater than 50% of thethickness, measured in the radial direction at the end of what in ameridian section of the tire is the radially outermost working layer, ofthe first layer of blended elastomeric compound.
 15. The tire accordingto claim 8, wherein the tire is of a dimension strictly greater than 57inches, wherein the thickness, measured in the radial direction at theend of what in a meridian section of the tire is the radially outermostworking layer, of the second layer of blended elastomeric compound isgreater than 10% of the thickness, measured in the radial direction atthe end of what in a meridian section of the tire is the radially